Use the dib python to do cleanup

In a system where python2 is not installed and /usr/bin/python is not
linked then the cleanup process will fail trying to invoke the python
script. Use the previously determined DIB_PYTHON_EXEC if it's available.

Change-Id: I128292808ccef92cc1803988b35caae5aa6fa541

diskimage_builder/lib/common-functions

@@ -154,7 +154,7 @@ function _get_chroot_processes () {
     # Deselect kernel threads, and use a python script to avoid
     # forking lots and lots of readlink / grep processes on a busy
     # system.
-    ps --ppid 2 -p 2 --deselect -o pid= | xargs python -c '
+    ps --ppid 2 -p 2 --deselect -o pid= | xargs ${DIB_PYTHON_EXEC:-python} -c '
 import os
 import sys